SUBJECTIVE ANTEDATING OF A SENSORY
EXPERIENCE AND MIND-BRAIN THEORIES: REPLY TO HONDERICH

by Benjamin Libet
--The Determinism and Freedom Philosophy Website
--
This
is Benjamin Libet's response to a critical paper by Ted Honderich about
neuroscientific research by Libet and colleagues and the philosophical
use made of the research. This was by Libet himself and also by Karl
Popper and J. C. Eccles in their book The Self and Its Brain.
Honderich's critical paper, to which you can turn, is Is
the Mind Ahead of the Brain? -- Benjamin Libet's Evidence Examined. For
Honderich's rejoinder to Libet's response below, see in turn Is
the Mind Ahead of the Brain? -- Rejoinder to Benjamin Libet. The research
and interpretation under discussion, having to do with sensations, was
followed by more Libet research of a different kind, about actions. For
a brisker paper that discusses both periods of research, there is
Honderich's Is the Mind
Ahead of the Brain? Behind It?, from the book On Determinism and
Freedom (Edinburgh University Press, 2005).

Abstract:
Honderich claims that our "delay-and-antedating" hypothesis, of a delay
in cerebral production combined with a subjective antedating of a
conscious sensory experience, involves self-contradiction, which may
cast doubt on some of our experimental findings and on the hypothesis.
This claim misses the distinction between the phenomenological
subjective mental content of an experience and the physical-neuronal
configuration that elicits the experience; also, it cannot explain the
experimentally observed discrepancy, between subjective timing and the
empirically delayed time for cerebral adequacy for eliciting the
experience, found when stimulating a subcortical sensory pathway.
Honderich usefully distinguishes between our stated
(delay-and-antedating) hypothesis and a different though unac-ceptable
one which would have serious implications for mind-brain theories. The
delay-and-antedating hypothesis does not provide a formally definitive
contradiction of monist-identity theory (of the mind-brain
relationship). However, our experimentally based hypothesis does
dissoci-ate subjective/mental timing from the actual physical/neuronal
time of an experience. This phenomenon, though conceptually strange,
must be encompassed by any mind-brain theory.

Introduction

Honderich reviewed our experimental findings
and analyzed our hypothesis on the relation of subjective timing of a
conscious sensory experience to its neuronal production (Libet et al.,
1979). The hypothesis states: (a) Delays of up to 0-5 sec or more occur
before achieving cerebral "neuronal adequacy" for eliciting the sensory
experience. (b) However, there is a subjective referral or antedating
of the experience back to a time close to the early initial signal,
arriving in 10-20 msec via the fast specific projection pathway and
represented by the primary evoked potential at the sensory cortex;
subjectively, then, there would appear to be no delay of the experience
(Libet et al., 1979; Libet, 1978, 1981, 1982). This is referred to by
Honderich as the delay-and-antedating hypothesis.
Honderich claims that, although we
clearly preferred the
delay-and-antedating hypothesis, a second hypothesis was implied by our
language and work, a so-called "no delay" hypothesis. In the latter,
there would be the same delay of up to 0-5 sec for neuronal production
of the experience, but the actual experience appears before then, with
no delay. He suggests that our failure to distinguish the two ditlerent
hypotheses may cast doubt on some of the experimental findings and led
us to an unwarranted view of certain consequences of the work for
mind-brain theories. I shall attempt to clarify my statements and views
on these issues.
Additionally, Honderich argues that our delay-and-antedating hypothesis
is itself doubtful and perhaps unacceptable. I shall aim to show that
his argument is based on a misapprehension of the significance of some
of the experimental findings as well as on a debatable philosophical
assumption he makes about the relation between subjective phenomena and
the corre-lated physical, cerebral events.
1. The Experimental Discrepancy
between Timings

First let us be clear about what the
experiments actually tell us.
Subjects reported the relative timing order of two sensory experiences,
i.e. reporting which one was felt first. The times of delivering the
two stimuli and the required minimum durations of each stimulus for
eliciting its sensory experience were known to the observer, but not to
the subject. From these (and accessory) data, inferences were made
about subjective timing of a sensory experience relative to the minimum
times required to produce the experience (i.e. to achieve a state of
neuronal adequacy for eliciting it) (Libet, 1973, 1981, 1982; Libet et
al, 1979). In discussing and diagramming the data we tended to place
the subjective experience at the times reported by the subject, these
reports being the only available directly valid indicators of such
times. Strictly speaking, the reported time represented the subject's
recalled impression of when the experience occurred, relative to
another sensory experience, and not necessarily the time of actual
appearance of the experience.* Although this qualification is clearly
implicit in our treat-ment, indeed it forms the basis of the
delay-and-antedating hypothesis, it was not always explicit in our
presentation. I believe this potential ambiguity of meaning in our
operational usage of experimentally determined terms led to some of
Honderich's perceived difficulties in our work.
Honderich (1984) provides a reasonable listing of our findings, but his
interpretation of their significance is incomplete and partially
misguided, particularly for his subset # 2 of our findings. In our
finding listed as # 2.1 by him, the experience of a skin stimulus was
reported by the subject to begin well before the experience for a
cortical stimulus, when both stimuli were started simultaneously; the
cortical stimulus empirically required about 0.5 sec of repetition to
produce any sensation, while the adequate skin stimulus was a single
brief pulse. This experiment was done to test our previous hypothesis
of "simple delay", namely that awareness of all sensory inputs would
require a delay similar to that for the cortical stimulus. Finding 2.1
was indeed a crucially initial one in that it appeared to contradict
our simple delay hypothesis. But we had previously obtained strong
though indirect evidence that a delay, of up to 0.5 sec to achieve
neuronal adequacy for eliciting the sensory experience, did occur even
for a skin stimulus (listed in Libet, 1973, 1978, 1981 ; Libet et al.
1967, 1972, 1979, but not detailed here). This paradox led us to
propose the delay-and-antedating hypothesis; retaining the requirement
of a neuronal delay, the early subjec-tive timing of the skin sensation
was now explained as a subjective referral or antedating of the
actually delayed experience backwards to the time of the initial fast
ascending signal.
The crucial experimental test of the delay-and-antedating hypothesis
lay in matching the time order of experiences for a skin pulse and a
subcortical stimulus (in the medial lemniscus pathway, LM); these are
listed as findings 2.2-2.4 by Honderich, who missed their full
significance. The LM stimulus, like a cortical one, requires the same
substantial period of repetitive pulses to become effective; with
intensity adjusted so that at least 0.2 sec of repetitive pulses (at
20-60 per sec) was required to elicit any sensation, we knew that the
cerebral production of the sensory experience had to be delayed for at
least that 0.2 sec. However, unlike the cortical stimulus, each LM
stimulus pulse in the train produces the same fast projection response
(the primary evoked potential) at sensory cortex that is elicited by a
skin stimulus, thus providing the putative early signal needed for
subjective antedating of the experience. The experimental test, which
could potentially have falsified the delay-and-antedating hypothesis,
in fact confirmed it. When the onset of the LM stimulus train was
simultaneous with a skin stimulus pulse, subjects reported that both
sensory experiences began simul-taneously (Libet et al., 1979). That
is, the LM-elicited sensation was experienced with no delay relative to
that for a single skin stimulus pulse, even though the LM stimulus
could not have become adequate until 0.2 sec had elapsed; an LM
stimulus train shorter than 0-2 sec elicited no sensation whatsoever!
This extraordinary finding, taken together with the different result
when matching onset of a cortical stimulus with a skin pulse (finding
2.1), provides conclusive experimental proof of a discrepancy between
the subject's timing of his experience and the delayed time at which
the LM stimulus became adequate. It seemed warranted to assume that a
similar discrepancy exists for skin stimulus, in view of the supporting
indirect evidence; the alternative possibility, that the
delay-and-antedating hypothesis applies specifically and only to the
subcortical LM stimulus, is unattractive and unnecessary.
In view of the foregoing, Honderich's argument (near the end of his
section 1), that finding 2.1 is itself either false or may make other
finding; entailing the delay-and-antedating hypothesis false, is based
on a faultv premise. Finding 2.1 does not provide crucial evidence for
a no-delav hypothesis; it can be satisfactorily and even more
attractively encompassed (with the added indirect evidence) by the
delay-and-antedating hypothesis. It should be emphasized that the
discrepancy between subjective and neuronal timings, directly observed
when testing with LM stimuli, derives from experimental observations
which have not been challenged on tech-nical grounds; it cannot be
refuted as a finding simply by some differing theoretical argument or
concept.
2. Subjective Content and
Neuronal Configuration

The distinction, between the phenomenological
subjective content of an
experience and the observable physical feature.s of the neuronal
activities that give rise to or are associated with the experience, is
a general one for all mind-brain relationships. Discrepancy between the
spatial features of a sensory experience and the spatial pattern of the
neuronal activity eliciting the experience has long been recognized and
spoken of as "subjective referral in space". For example, in a visual
experience the image is not only projected into the external
environment, not located at its neuronal site of origin in the brain,
but the subjective configuration of the image differs drastically from
the underlying neuronal patterns. There is in addition the
neurologically well-known phenomenon of subjective "filling in" of the
missing portion of a blind area in the visual field.
Our findings in support of the
delay-and-antedating hypothesis led to
an analogous concept in the temporal dimension. In temporal referral
the sensory experience is antedated or subjectively referred back to a
time preceding the delayed time at which neuronal adequacy is achieved;
in spatial referral, analogously, the spatial form of a somatosensory
experience, for example, is subjectively referred to a part of the body
and in a pattern quite different from the spatial configuration of
neuronal activity in the cerebral cortex. Our further evidence
supported the view that the same ascending pathway to sensory cortex,
the specific projection system (lemnis-cal for the somatosensory
modality), provides the signal utilized for both types of referral.
Honderich dismisses this clear analogy solely on the basis of his own
assumption that the discrepancy between subjective and neuronal
timings, in temporal referral, involves simultaneous,
self-contradictory impressions (see below).
Indeed, we have no way of predicting a priori, from a knowledge of the
neural basis of an experience, what the content or nature of a
subjective experience will be, even if one assumes the relationship
between the "phys-ical" (externally observable) and the "mental"
(internally observable) is one of psychophysical lawful correlation
(see Libet, 1982; Nagel, 1974). The experience can only be described in
an introspective report by the subject. Honderich appears to violate
this principle when he insists that he knows, a priori, what must be
the subject's impression of the timing of an experience.
Honderich insists it is "surely true that the actual having of any
conscious sensory experience -- is accompanied by the belief or
impression that the experience-is happening now." He claims that our
postulated "antedating or referring back (of the experience in time)
involves imputing something like simultaneously-held, fully explicit
self-contradictory beliefs to subjects...". But Honderich appears to
confuse the content of an experience with the actual time of its
appearance. We do not regard the subject as being unaware of his
experience at the actual time it appears, but we accept the subject's
report of his impression of when he felt the experience appeared, and
we attempt to account for the discrepancy. At whatever time an
experience occurs, the subject's impression of when it appeared could
depend on other modulating factors. In our hypothesis, the early,
primary cortical response, evoked by the fastest arriving neural
component initiated by the sensory input, serves as the signal that
leads to a temporal modulation of the subject's impression of when the
experience occurred; the localized pattern of these same responses
incidentally also serves to modulate the subject's spatial impression.
With the stimulus located on somatosensory cortex where it does not
elicit this special primary evoked response, the subject reports a
delayed time of experience that does in fact coincide with the delay
required for the stimulus to become effective.
Contrary to Honderich's belief, the subject is not forced by the
hypothesis to have two contradictory impressions or beliefs about when
he felt the experience; he would only have one impression of the time
he felt the experience. Operationally, the subject in fact reports an
awareness of only one sensory experience with a single time onset. The
subject has no aware-ness of any discrepancy between his subjective
impression, of when he had the experience, and the actual time of the
experience (when neuronal adequacy for producing it occurs). To the
extent that some kind of neuronal retention of the early ascending
signal may be involved, such a memory process is completely unconscious
and not accessible to conscious aware-ness, unlike ordinary memories of
events or experiences. The same uncon-scious automaticity applies to
subjective referral in space; we are all, as "subjects", completely
unaware of the discrepancy between our neuronal spatial patterns and
the subjective image they produce. Interestingly, however, the
relationship between image and neuronal pattern can be altered
by.re-learning.
Honderich's apparent demand for an "account of the supposed phenomenon"
of subjective referral backwards in time, or a description of "what
processes" mediate the unconscious and automatic referral, is some-what
strange. One can only describe relationships between subjective
phenomena and neural events, not how one gets from one to the other.
The critical components of the neural correlates in the
delay-and-antedating hypothesis are, in fact, as or more complete than
is the case for most mind-brain correlations.
Finally, subjective referrals in space or time are fundamentally
different from sensory illusions. In illusions there are distortions of
the real stimulus configurations, whereas subjective referrals serve to
"correct" the experience so as to be closer to the actual time or form
of the sensory stimulus than is the neuronal timing or spatial form
(Libet, 1981). Honderich appears to prefer the suggestion by Donald
MacKay that subjective antedating may be due to an illusory judgment
made by the subject when he reports the timing (discussed by Libet et
al., 1979). But, as already noted (Libet et al., 1979) such a proposal
entails a serious difficulty, aside from arguments about simplicity of
hypotheses. "By retaining delays for the immediate subjective sensory
experiences, when they initially and actually occur, this alternative
suggestion of MacKay's becomes unable to explain the absence of
subjective "jitter" or asynchrony in our experience, when a variety of
peripheral sensory stimuli are applied synchronously. At least one
factor that should produce differences in the delays for achieving
neuronal adequacy with different stimuli, is the strength of the
stimulus. (See dis-cussion in Libet et al., (1979) and Libet, (1981).)
One attractive feature of our (delay-and-antedating) hypothesis is in
fact its ability to deal with this difficulty. Subjective referrals,
that are retroactive to the early primary evoked response to each
sensory input, would make irrelevant any differen-ces among the timings
for neuronal adequacy in a group of synchronously initiated inputs;
delays for the primary evoked potential are short (10 to 20 msec), and
the differences produced by differing intensities of peripheral somatic
stimuli are known to be so small as to be negligible for the purpose of
subjective timing" (Libet et al., 1979, p. 220).
3. Import for Mind-Brain Theories.

Honderich (1984) rightly points out that a
no-delay hypothesis would
flatly threaten identity theory and psychophysical lawlike correlation,
but that such a hypothesis "must be false, in that a mental item occurs
before the physical item on whose later existence it depends". We did
not espouse such a hypothesis, as Honderich concedes, but some of my
language in the earlier papers led him to a conjecture that we were
unwittingly adopting this hypothesis as a basis for our conclusions
about the mind-brain relation. I had previously stated that our
findings of a discrepancy between subjective timing of an experience
and the time of its neural production (i.e. the delay-and-antedating
hypothesis) "is not any denial of correspondence between mental and
physical events, but (is) rather (a description of) the way in which
the correspondence is actually manifested" (Libet, 1981, p. 195); and
that "a discrepancy between the `mental' (i.e. subjective timing of an
experience in this case) and the `physical' in the temporal dimension
can be regarded, in a manner analogous to that for the discrepancy in
the spatial dimension, as not contradicting the theory of
psychophysical parallelism or correspondence" (Libet et al. 1979, pp.
221-222).
I did add that such a "temporal discrepancy
creates relative
difficulties for identity theory, but that these are not
insurmountable" (Libet, 1981, p. l96). Honderich disagrees with this,
stating that our delay-and-antedating hypothesis presents "no problem
whatever for an identity theory" (his section 4). The kind of
difficulties I referred to is perhaps exhibited in part by Honderich
himself, in his expression that the subjective antedating of a sensory
experience "may indeed be of a strange self-contradictory kind".
Although the delay-and-antedating hypothesis does not separate the
actual time of the experience from its time of neuronal production, it
does eliminate the necessity for simultaneity between the subjective
timing of the experience and the actual clock-time of the experience.
But we find Honderich insisting that the opposite is true, namely that
there must surely be a simultaneity between the subject's impression of
an experience and the actual time of the experience!
Many will concede that an absence of
isomorphism, say for a visual
image relative to its corresponding neuronal pattern, is acceptable to
and compatible with identity theory. On a strictly formal basis the
same may be said for the temporal discrepancy, as expressed in the
delay-and-ante-dating hypothesis in which simultaneity of the actual
experience and its neuronal counterpart is preserved; this is also
Honderich's position. To the extent that my previous statements may be
taken by Honderich and others to imply a different position, I would
agree that such an implication should be removed.
However, the phenomenon of subjective referral of a sensory experience
backwards in time is indeed a strange and not intuitively obvious
concept (Libet et al. 1979; Libet, 1981) (although, as I argue above,
it is not self-contradictory). In it, subjective mental timings of
experiences become dissociated from actual clock-times of the
experiences. It shows that subjec-tive phenomena "can play tricks with
time", as Eccles aptly put it (Popper & Eccles, 1977, p. 364),
although admittedly these tricks are accomplished unconsciously and in
relation to specifiable neural information. To the extent that
adherents of a monist-identity theory may be unconfortable with such
subjective "trickery", they could have informal subjective (rather than
formal objective) difficulties in accepting the antedating phenomenon,
even though there is a solid experimental basis for the temporal
discrepancy in the timings.
--------------------
The above paper appeared in the Journal
of Theoretical Biology (1985) 114, 563-570. I am very grateful
to Professor John Searle of the Department of Philosophy, University of
California, Berkeley, for helpful comments on an earlier draft of the
manuscript, though responsibility for all stated positions is mine.